Master Chief gets in a covenant ship and heads for Earth.
The Arbiter teams up with some Commando's to retrieve the key (The Index)
from the Brutes.
Big fight and the Brutes activate Halo.
Then Amanda Keyes *the daughter of Captian Keyes from the first game) grabs
the key and stops the Halo fring sequence. PHEW !!
But this triggers the remaining 5 Halos dotted about the universe into
"Standy Mode" where they can be remotely detonated.
The detonator is on "The Ark".
The Ark = Earth !!
Master Chief enters Earths orbit in the Alien craft and some general asks
him what he's doing in an alien craft,
He says..."Coming to finish the job"